Build Instructions

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• 1

  Using PWM with Arduino

  Alright, we have two goals in this tutorial, just like in the parallel SDK and Lua tutorials: we're going to learn how to use PWM to fade an LED, and we're going to use I2C to communicate with a chip. Make sure to read the details section first if these terms don't make sense to you; I go into a detailed description of how PWM and I2C work.

  PWM is very easy in the Arduino environment. Arduino has a built-in function called analogWrite() which creates a PWM output on pins which support it. Again, for all these examples I will be using an Adafruit HUZZAH Feather, but they will work just fine with any ESP board. I recommend avoiding the ESP-01 for development. Use something like the ESP-12F instead, it's much easier. On the Feather, we will be using GPIO 0 and 2, which are the two on-board LEDs. On other boards, just check the documentation to see what pins have LEDs on them. You can, of course, attach an external resistor and LED if you want.

  The analogWrite() function takes two parameters: the pin we want to use for PWM output, and the duty cycle value. The duty cycle is a 10-bit value, from 0-1023. To get a specific percentage, just multiply the percentage you want by 1024 (the number of values). If we wanted 30% duty cycle, for example, we would use:

  Note, however, that 30% duty cycle is not the same as 30% brightness. The relationship between current and brightness is non-linear in LEDs. This means most of the dynamic range of an LED's brightness is near the low end of the 10-bit scale. There are many ways to scale the output to make it look smoother to the human eye. We will take a quick look at this later on.

  We're going to use a very simple state machine to fade our LEDs up and down. The state machine can have two states: increasing or decreasing. The duty cycle increases from 0 to 1023. When it hits 1023 the state changes to decreasing, and it heads back down to 0. This loops indefinitely. It's one of the simplest ways to fade an LED! Let's take a look at the code.

• 2

  The "Simple Fade"

  Our first test program will be using a very simple state machine algorithm, which, as described, simply keeps track of the fade direction and modifies the duty cycle as required.

  /*
   * This sketch is written by MrAureliusR (Alexander Rowsell)
   * It it released under the terms of the Mozilla Public License v2 (https://www.mozilla.org/en-US/MPL/2.0/)
   * It is part of the ESP8266 Tutorial Series on Hackaday! Make sure to read the accompanying article. 
   * 
   * We will be using the built-in analogWrite function to do our PWM for us!
   * 
   */
  
  enum {DECREASING = 0, INCREASING = 1};
  
  int dir = DECREASING, stepSize = 1, curStep = 0, led = 2, delayTime = 10;
  
  void setup() {
    Serial.begin(115200);
    pinMode(led, OUTPUT);
  }
  
  void loop() {
    if(curStep == 0) dir = INCREASING;
    else if(curStep >= 1024) dir = DECREASING;
  
    if(dir == INCREASING) {
      analogWrite(led, curStep);
      curStep += stepSize;
      if(curStep >= 1024) curStep = 1024;
    } else if (dir == DECREASING) {
      analogWrite(led, curStep);
      curStep -= stepSize;
      if(curStep <= 0) curStep = 0;
    }
    Serial.print("Current step: ");
    Serial.println(curStep);
    delay(delayTime);
  
  }

  To make things simple, we declare a few variables for things like the LED pin, the current duty cycle, the amount to change the duty cycle in each loop, and how long to delay at the end of the loop. I've used an enum instead of #defines for increasing and decreasing. This is a good habit to get into, but it doesn't really matter which you use for such a small program.

  We also start the serial output up, so we can check the progress of the fade, though this can be commented out and is mostly just for debugging. Even beginners should be able to grasp what is happening in this code. If you have questions or are confused, feel free to ask in the discussion below, or join us on Freenode IRC in ##esp8266.

  Next, we'll take a look at a little trick to make the fade look a bit smoother.

• 3

  The "Smooth Fade"

  The human eye does not perceive light output linearly in relation to LED current. What this means is to get twice the perceived brightness, you often need to use about four times the current. LEDs function most efficiently below their max current, so making the most of the dynamic range we have is essential. Mike Harrison of mikeselectricstuff explains this really well in this video. The simplest way to do this is to take the square of the output we want. The square root of 1024 is 32, but I take the input up to 50 which gives us a bit more time at max brightness. Compare this with the previous sketch and see which one looks better to you!

  /*
   * This sketch is written by MrAureliusR (Alexander Rowsell)
   * It it released under the terms of the Mozilla Public License v2 (https://www.mozilla.org/en-US/MPL/2.0/)
   * It is part of the ESP8266 Tutorial Series on Hackaday! Make sure to read the accompanying article. 
   * 
   * We will be using the built-in analogWrite function to do our PWM for us!
   * 
   */
  
  enum {DECREASING = 0, INCREASING = 1};
  
  int dir = DECREASING, stepSize = 1, curStep = 0, outStep = 0, led = 2, delayTime = 50;
  
  void setup() {
    Serial.begin(115200);
    pinMode(led, OUTPUT);
  }
  
  void loop() {
    if(curStep == 0) dir = INCREASING;
    else if(curStep >= 50) dir = DECREASING;
  
    if(dir == INCREASING) {
      outStep = pow(curStep, 2);
      if(outStep >= 1024) outStep = 1024;
      analogWrite(led, outStep);
      curStep += stepSize;
      if(curStep >= 50) curStep = 50;
    } else if (dir == DECREASING) {
      outStep = pow(curStep, 2);
      if(outStep >= 1024) outStep = 1024;
      analogWrite(led, outStep);
      curStep -= stepSize;
      if(curStep <= 0) curStep = 0;
    }
    Serial.print("Current step: ");
    Serial.print(curStep);
    Serial.print("\tOut step: ");
    Serial.println(outStep);
    delay(delayTime);
  
  }

  Again we have serial debugging just so we can easily see where in the cycle we are. Now that we've mastered PWM, let's move on to the slightly more complex I2C serial bus. Make sure you've fully read the details section so you have a basic understanding of how the I2C bus works.

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